A deoxyribonucleoprotein complex extracted from Simian Virus 40 (SV40) infected monkey kidney cells and from SV40 virions is used as a model system for the study of chromatin structure and function. Recent experiments in this laboratory have demonstrated a pattern of nuclease-sensitive sites which map uniquely in the SV40 genome. These results imply the existence of a unique chromatin structure located in a region of the virus genome that is intriguing because of its biological importance -- it contains the origin for SV40 DNA replication and, probably, promoters and control elements for viral RNA synthesis. Experiments are proposed to determine the location and nature of the nuclease-sensitive sites in SV40 chromatin, and to map the SV40 genetic element(s) responsible for organization of this structure. The nuclease-sensitivity pattern will provide a valuable assay to evaluate maintenance of chromatin structural fidelity during extraction and purification. Attempts will be made to subfractionate SV40 chromatin, isolate the segment containing the region of interest, and characterize histones, histone modifications and other proteins associated with this region. Additional experiments are proposed to reconstitute chromatin from its isolated components and evaluate the recovery of the nuclease-sensitivity pattern. SV40 mutants will be used to decide whether a viral gene product is involved in this structure. Other physiological manipulations are proposed to determine what chromatin configurations are present at different stages of viral infection. Finally, isolation procedures are described to recover transcription and replication complexes as chromatin subfractions to decide whether functionally active chromatin structures have altered nuclease-sensitivity patterns. This project will contribute to our understanding of relationships between the structure of eucaryotic chromatin and control of gene expression and can lead to new information about human disease mechanisms.